Water pollution is gradually serious, so it is necessary to treat various sewages economically and effectively. Currently, thousands sewage treatment plants are running in the world, and more sewage treatment plants will be built with the development of environmental senses and requirements.
Sewage biotreatment is widely applied to sewage treatment due to its outstanding advantages of high performance and low energy consumption, and sewage biotreatment technologies represented with activated sludge technologies and biomembrane technologies are successfully used in sewage treatment and pollution abatement. However, the sewage biotreatment technologies in the prior art still have some drawbacks.
In practical processes, many sewage plants have to face the following challenges: (1) insufficient amount of sewage feed, mainly resulted from advanced design and failures in sanitary sewer system, which may influence the operation of sewage treatment apparatus; (2) unstable quality of sewage feed, mainly resulted from draining industrial wastewater into sanitary sewer system and living habit changes caused by holidays and seasonal variation, which may form shock load to influence sewage treatment effects; (3) carbon-source deficiency, as a common problem of sewage treatment plants, mainly attributed to modern living habits, which may result in unbalance of nutrients in organisms and influence the removal effects of nitrogen and phosphorus. Facing these challenges, conventional activated sludge methods usually have the following drawbacks: (1) low biomass concentration in aeration basin; (2) poor ability in resisting shock load of quality and amount of sewage feed; (3) easy sludge bulking; (4) high yield of sludge; (5) high cost in construction and operation, and large floor space, etc.
In particular, conventional sewage biotreatment technologies usually have an arresting problem of producing a large amount of sludge. The cost for treating sludge usually is dramatically high, and is about 50-60% of the total cost for construction and operation of a sewage treatment plant. Residual sludge needs to be treated and therefore increases the operation cost and limits the choice of activated sludge treatment methods. In general, sludge reduction methods include digestive methods (including anaerobic digestive methods and aerobic digestive methods), sludge heat-treatment method such as wet oxidation methods, sludge concentration methods such as gravity thickening methods and air-floating methods, sludge dewatering methods such as mechanical dewatering methods and chemical aggregating methods, sludge anhydrating methods such as natural anhydrating methods and oven drying methods. However, these methods are still not enough to address the problems of sludge discharge.
Chinese patent application CN10148119A discloses a sewage treatment method with a backflow of sludge for digestion and reduction, in which a residual sludge returns back an anaerobic sedimentation basin and accumulates at the bottom of the anaerobic sedimentation basin for a long period of time so as to digest and reduce the sludge, while a sewage feed passes through the anaerobic sedimentation basin and then is subjected to a sewage treatment to obtain a purified water and the residual sludge, in which the undigested sludge must be removed periodically.
U.S. patent application US2002/0030003A1 discloses an activated sludge system and method, in which wastewater is treated with sludge in a contact tank, then sludge and water are separated in a solid/liquid separator, the separated sludge and a part of wastewater are mixed and aerated in a digester basin to digest and reduce sludge, the aerated mixture of sludge and water is partially returns to the contact basin and partially discharged.
In addition, conventional biotreatment of sewage usually is not good at the removal of phosphorus. Phosphorus is the main restriction factor causing eutrophication of water body, and is an important element for sustainable development of human being. Hence, the control of phosphorus content in water body is gradually stringent, and is changing from single “removal” to “recovery”. Currently, dephosphorization technologies are mainly based on phosphorus-accumulating bacteria which can uptake and accumulate a large amount of phosphorus under aerobic condition after releasing phosphorus under anaerobic condition. Hence, a certain amount of sludge is still discharged to finally remove phosphorus, which also challenges the reduction of sludge.
In sum, novel sludge treatment methods and sewage treatment methods are still in need to be development so as to address the above problems, especially the problems about sludge reduction.